Process for preparing polysilicon with diminished hydrogen content by using a two-step heating process

ABSTRACT

The hydrogen content of polysilicon can be reduced by heat treatment. The process is preferably conducted on polysilicon particles in bead-like form produced by chemical vapor deposition in a fluidized bed. The heat treatment is preferably conducted at a temperature of 1020°-1200° C. for a time from about 6 hours to about 1 hour sufficient to reduce the hydrogen content, and insufficient to cause agglomeration of the particles being treated. In order to reduce the tendency of particles to agglomerate at the process temperature employed, the particle bed is preferably maintained in motion during the dehydrogenation. The products produced by the process can have a hydrogen content of 30 ppma or less. These improved products can be used to produce monocrystalline silicon for the production of semiconductor devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 07/791,882,filed Nov. 13, 1991 now U.S. Pat. No. 5,242,671 which in turn is acontinuation-in-part of application Ser. No. 07/255,967 filed Oct. 11,1988, now abandoned. Reference is made to two U.S. Patents filed by M.F. Gautreaux and Robert H. Allen. They relate to a fluidized bed processfor producing polysilicon and the products produced thereby. The U.S.Patents are U.S. Pat. Nos. 4,784,840, and 4,820,587.

BACKGROUND OF THE INVENTION

Polysilicon of the type disclosed in the above-identified applicationsis composed of free flowing, approximately spherical particles. Theseparticles can be transported and handled readily. Hence, such bead-likeproducts offer crystal growers a product that is tailor-made fordevelopment of continuous melt replenishment systems used in productionof monocrystalline silicon. Monocrystalline silicon is used in theproduction of semiconductor devices.

For a description of the polysilicon and process provided by Messrs.Gautreaux and Allen, the above-cited applications are incorporated byreference herein, as if fully set forth.

This invention relates to an upgrading improvement in the polysilicondisclosed in the above-mentioned applications. In a continuing effort toimprove such polysilicon, it has been discovered that such product canbe improved by a heat treatment which reduces the content of a volatileimpurity, which is believed to be hydrogen.

Applicants are unaware of any prior art relating to hydrogen removalfrom polysilicon. Sanjurjo et al, U.S. Pat. No. 4,676,968 discloses aprocess for melt consolidating polysilicon powder. For meltconsolidation, the powder is heated at a temperature above the meltingpoint of silicon. (1410° C.). The processes of this invention do notemploy temperatures above silicon's melting point. Furthermore, anobject of this invention is to produce heat treated polysilicon whileavoiding melt consolidation. Accordingly, the processes of thisinvention markedly differ from the process of the Sanjuro et al patent.

Flagan et al., U.S. Pat. No. 4,642,227, discloses a method for producinglarge particles of materials, and more particularly, to a free spacereactor for producing particles greater than a few microns (preferablyin the range of 10-100 microns). The processes of this invention do notemploy the residence times for maintaining polysilicon particles attemperatures which reduce the hydrogen concentration as seen in thepresent invention.

SUMMARY OF THE INVENTION

In a particular aspect, this invention relates to fluidized bedproduced, semiconductor grade polysilicon having a hydrogen contentreduced by a heat treatment. The heat treatment to reduce the amount ofhydrogen impurity may be conducted using various techniques, forexample, it may be conducted using a moving bed or a fluidized bed.

In a preferred embodiment, the hydrogen content of the upgraded productprovided by this invention is about 30 ppma or less. Typically, theimproved products of this invention are in bead-like form. In otherwords, the products of this invention are in the form of approximatelyspherical particles. In general, these particles may have a size rangefrom 150 to 3000 microns. Preferably, these particles have a size rangeof from about 400 to about 1500 microns; and have an average size offrom about 600 to about 1200 microns. Such products are free-flowing andreadily transported and handled. Thus, they are also eminently suitedfor continuous and semi-continuous processes for producing semiconductorgrade monocrystalline silicon, and especially suited for such processeswherein hydrogen impurity is a problem.

FIG. 1 illustrates the reduction in hydrogen content obtained by heatingsamples of polysilicon beads. As shown, one sample was heated at 896°C.; the other was heated at 1091° C. Each of the polysilicon samplesweighed about 90 grams.

The samples were heated in a vertical quartz tube about 1 inch indiameter and several inches long. Prior to heating the samples, the tubewas heated at 225° C. overnight to remove moisture and/or othermaterials which could interfere with the analysis. The weight of thepolysilicon samples charged to the tube were within the range of 85-100grams. The size of the tube allowed good contact between the beadsurfaces and the gas phase. The tube was fitted with a thermowell, inwhich a thermocouple was inserted to permit accurate temperaturemeasurement.

The tube and contents were heated to the temperature causing thepolysilicon to outgas. The evolved gas was pumped to a reservoir. Atintervals, (typically every ten minutes) the temperature and pressure ofthe reservoir, and of the sample tube were noted, allowing thecalculation of the amount of gas released during the interval.

Some agglomeration of the beads was noted, particularly in the sampleheated to 1091° C.

The initial concentration of hydrogen in the polysilicon was about 620ppma. In the drawing, the percent hydrogen remaining at time zero isless than 100% because some hydrogen was removed during the period(approximately 30 minutes) required for the sample tube and contents toreach the test temperature. The circled points in the drawing representdata obtained from the following table. The curves show trends of thedata at both temperatures.

                  TABLE I                                                         ______________________________________                                        Polysilicon Dehydrogenation                                                                  Fraction of                                                    Time           Hydrogen Remaining                                             (minutes)       396° C.                                                                        1091° C.                                       ______________________________________                                         0             0.6178   0.6076                                                 10            0.5717   0.2816                                                 20            0.5306   0.1920                                                 30            0.4963   0.1505                                                 40            0.4656   0.1238                                                 50            0.4389   0.1063                                                 60            0.4158   0.0923                                                 70             0.3927* 0.0823                                                 80            0.3757   0.0742                                                 90            0.3581   0.0742                                                100            0.3424   0.0689                                                110            0.3280   0.0629                                                120            0.3141   0.0589                                                130            0.3015   0.0551                                                140            0.2896   0.0515                                                150            0.2786   0.0494                                                160            0.2687   0.0471                                                170            0.2587                                                         180            0.2500                                                         190            0.2410                                                         200            0.2327                                                         210            0.2244                                                         ______________________________________                                         *71 minutes                                                              

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the reduction in hydrogen content obtained by heatingsamples of polysilicon beads.

FIG. 2 is a representation, partially in cross-section and not to scale,of a fluidized bed apparatus for use in this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

This invention relates to polysilicon having a volatile impurityremoved. From chromatographic evidence, applicants believe that theremoved impurity is hydrogen. However, the heat treatment used in theprocess of this invention may also remove other volatile impuritieswhich are present in the polysilicon product to be upgraded.

In a particular aspect, this invention relates to a polysilicon product(upgraded by heat treatment) which was produced in a fluidized bedprocess. Thus, the invention relates to improved polysilicon beads orbead-like particles. In a preferred embodiment, the polysilicon to beupgraded is a material which was produced in a fluidized bed by thethermal decomposition of silane.

In general, this decomposition of silane (SiH₄) is conducted attemperatures of from about 600° C. to about 750° C. In addition tosilane, hydrogen is liberated from the decomposing silane and becomesintegrated into the resulting silicon particles. Chlorosilanes, such asdichlorosilane and trichlorosilane may be decomposed at greatertemperatures to yield silicon particles with a lower hydrogen content.Such particles, however, present the undesirable quality of chlorinecontamination. The silane-derived particles of the present inventioncontain very little or no chlorine content.

Accordingly, in one embodiment, this invention relates tosilane-derived, fluidized bed produced, semiconductor grade polysiliconhaving a low hydrogen content. The low hydrogen content is achieved byheating the polysilicon for a time and at a temperature sufficient toremove hydrogen from the polysilicon. As an example, a polysiliconproduct produced in a fluidized bed by the thermal decomposition ofsilane may contain from about 100 to about 1000 ppma hydrogen. After theheat treatment conducted according to the process of this invention, thepolysilicon can have a hydrogen content of 30 ppma or less. The hydrogenremoval step can be conducted as an operation separate from andsubsequent to the formation of polysilicon in a fluidized bed.

Thus, this invention relates to a process for heating silane-derivedpolysilicon (preferably in the form of beads or bead-like particles) fora time and at a temperature sufficient to reduce the hydrogen content ofthe polysilicon. Thus, in another preferred aspect, this inventioncomprises a process for heat treating silane-derived polysilicon in theform of beads, preferably having a size within the range of from about150 to about 1500 microns, said process comprising heating saidpolysilicon in hydrogen at a temperature and for a time (a) sufficientto reduce the hydrogen content of said beads, and (b) insufficient tomelt consolidate said polysilicon beads. The beads are preferably keptin motion during the dehydrogenation period in order to diminish theamount of particle agglomeration. In general, agglomeration atcomparatively high dehydrogenation temperatures increases if theparticles are static, i.e. not in motion during the heating period.

Agglomeration tends to take place when the dehydrogenation temperatureis high enough to soften the polysilicon particles being treated. Forthis reason, temperatures above about 1200° C. are generally avoidedalthough slight excursions above that temperature can be tolerated insome instances. Generally, the process temperature is kept considerablybelow the melting point of silicon, 1410° C.

Thus, in still another preferred embodiment this invention relates to aprocess for dehydrogenating silane-derived polysilicon beads having asize range of from about 150 to about 3000 microns, preferably 400 to1500 microns, said process comprising heating said beads in a fluidizedbed at a temperature of from about 1020° C. to about 1200° C., and atambient pressure, for a residence time sufficient to reduce the hydrogencontent of said beads, said beads being maintained during said heatingin fluidized suspension by motive force supplied by a stream of gasselected from hydrogen and the gases of Group VIII of the PeriodicTable.

As indicated above, a fluidized bed method need not be used. Moving bedmethods can also be employed if desired.

For this invention, a preferred polysilicon for upgrading is a materialof the type disclosed in the above-cited applications of Gautreaux andAllen. Polysilicon products produced by the method disclosed in thoseapplications generally will contain some hydrogen. The hydrogen contentappears to be a function of the operating conditions employed. Generallyspeaking, it appears that the hydrogen content is inversely proportionalto the process temperature used. In other words, use of lowertemperatures for decomposition of silane, results in higher content ofhydrogen in the product particles.

Polysilicon produced by the fluidized bed method of Gautreaux and Allenis in the form of free-flowing, approximately spherical beads. Ingeneral, the size distribution of such starting materials has a range offrom about 150 to about 1500 microns. A typical size range is about 400to about 1500 microns. The particle density in grams per cubiccentimeter is typically within the range of 2.25 to 2.33; a typicalaverage is 2.30 to 2.31. Preferred materials have a bulk density ofabout 1360 kg/m³. Surface dust is typically less than 0.1 percent, e.g.0.010-0.070 weight percent. A typical material of this type which can beused as a starting material for this invention is a silane-derived,semiconductor grade, polysilicon characterized in that:

(i) it is in bead-like, approximately spherical form,

(ii) has a surface morphology illustrated by FIGS. 3 and 3A ofapplication Ser. No. 4,116,

(iii) has a size distribution of from about 400-1500 microns,

(iv) has a boron content within the range of 0 01-0.25 ppba,

(v) has a phosphorus content within the range of 0.01-0.19 ppba, and

(vi) has a carbon content within the range of 0.16-0.33 ppm,

said particles being in admixture with less than about 0.08 percent ofsurface silicon dust particles having a size up to 10 microns. Suchpolysilicon is free flowing, readily handleable and transportable, andsuitable for continuous melt replenishment systems for producingmonocrystalline silicon.

The average size of silane-derived, semiconductor grade polysiliconparticles is from about 600 to about 1200 microns; most preferablyaveraging between about 800 and about 1000 microns.

As stated above, the process of this invention can be conducted using apolysilicon containing from about 100 to about 1000 ppma hydrogen. It isto be understood, however, that this invention can be applied topolysilicon containing a greater or lesser amount of hydrogen impurity.There is no real upper limit on the amount of hydrogen in thepolysilicon to be upgraded. With regard to a lower limit, one does notuse as a starting material a polysilicon having a hydrogen content whichcannot be reduced by treatment at the operating temperature and timeselected. For example, as shown in the drawing of FIG. 1, the rate ofreduction in hydrogen content is not large after a certain operatingtime. Furthermore, at any given temperature there appears to be apolysilicon hydrogen content which cannot be reduced further for aneconomically feasible reaction period. Thus, the hydrogen content is aprocess criterion to be considered.

In this invention, the polysilicon to be upgraded is heated at atemperature and for a time sufficient to reduce the hydrogen content ofthe particles being treated, and insufficient to melt consolidate, i.e.agglomerate, the particles by sintering. It has been discovered thatprocess temperatures of from about 1020° C. to about 1200° C. result insatisfactory diffusion rates and practical reactor sizes. Highertemperatures greatly increase the sintering rate as the melting point ofsilicon is neared. Lower temperatures require impractical, i.e.uneconomic, reaction times.

Generally speaking, reaction times of less than about 10 hours arepreferred. More preferably, reaction time is within the range of fromabout 1 to about 6 hours, with most preferable times being in the rangeof from about 2 to about 4 hours. The reaction time is not a trulyindependent variable, but is dependent to an appreciable extent on thereaction temperature employed. In general, the higher the reactiontemperature the shorter the dehydrogenation time required.

The dehydrogenation of this invention proceeds well at ambient pressure;However, higher and lower pressures can be used if desired. In general,pressures higher than ambient retard the removal of volatile substancefrom the polysilicon particles being treated, and subatmosphericpressures facilitate the process. When selecting a subatmosphericpressure, cost considerations should be borne in mind. There is no reallower limit on the pressure employed, and an operator can select anysubatmospheric pressure desired, e.g. down to one torr or lower.

To facilitate contact of the polysilicon particles with the vapor phasein order to promote diffusion of hydrogen from the particles, and alsoto reduce the tendency of particles to agglomerate at the processtemperature employed, the particle bed is preferably maintained inmotion during the dehydrogenation process. Thus as stated above, one mayuse a moving bed or a fluidized bed apparatus. A preferred processutilizes a fluidized bed. Fluidized beds result in higher heat transferrates between solid and fluid compared to other conventional modes ofcontact (an important factor in high temperature operation).Furthermore, the smooth flow of fluidized particles promotes ease ofhandling.

Referring to the drawing, a fluidized bed reactor 10 can be employed inour dehydrogenation process. In the reactor is fluidized bed 10a,comprising polysilicon particles being upgraded by dehydrogenation. Thereactor has free space 10b above the fluidized bed. The bed is heated toprocess temperature by radiant heater 11 which surrounds reactor 10. Thebed of particles is maintained in a fluidized state by a stream ofhydrogen gas which enters reactor 10 through line 12. After entering thereactor near the base thereof, the stream or motive gas flows throughdistributor 13 into the particle bed. Distributor 13 has multipleorifices in order to distribute the flow of motive gas throughout thebed, and maintain it in fluidized suspension. Prior to entering thereactor, the motive gas can be heated by any suitable heating device(not shown). For example, the motive gas can be heated to a temperatureof 325° C. or above.

The motive gas exits the reactor through line 14 near the top thereof,and then flows into solids filter 15. Polysilicon dust which isentrained in the gas exit stream is removed by the solids filter.By-product solids are removed from the filter through line 16. Effluentgas with solids removed is discharged through line 17. If desired, theeffluent gas can be recycled (after purification in purification zone18, if necessary).

After dehydrogenation, the upgraded product is removed via line 19 toproduct cooler/collector 20. As required, dehydrogenated product can beremoved from the cooler/collector via line 21.

A preferred mode of operation of the fluidized bed is semi-batch inwhich about 10 to about 20 weight percent of the total bed mass iswithdrawn and a similar quantity of untreated material is charged everycycle (typically 1 to 3 hours). After the withdrawn product is cooled toa suitable handling temperature, approximately 50°-60° C., the hydrogenatmosphere surrounding the withdrawn pellets is replaced with an inertgas such as argon, and the polysilicon product appropriately packaged toprevent or substantially prevent contamination. The packaging isconducted under an argon atmosphere.

It is not necessary to use hydrogen as the motive gas; other gases canbe employed if desired. When hydrogen is used as the motive gas,conflicting hydrogen diffusion mechanisms occur. On the one hand,hydrogen diffuses out of the particles being treated. At the same time,hydrogen in the gaseous phase near the polysilicon beads tends todiffuse into the particles. The net result of these two opposingmechanisms determines the overall effectiveness of the dehydrogenationtreatment. The opposing mechanisms are discussed below.

At any given process temperature employed, hydrogen within thepolysilicon particles has a certain tendency to diffuse out of theparticles and into the surrounding gas phase. The hydrogen diffusionrate is dependent, at least to some extent, upon (a) the concentrationof hydrogen in the particles and (b) the microscopic structure of thepolysilicon particles. Some hydrogen diffuses out of the particles bytraveling between the silicon atoms in the crystalline matrix. Otherhydrogen tends to leave the particles via voids or interfaces betweencrystalline surfaces in the particles.

On the other hand, hydrogen within the gaseous fluid surrounding theparticles has a tendency to diffuse into the particles. This tendencydepends at least to some extent on the hydrogen concentration in the gasphase. As the concentration of hydrogen increases, the rate of hydrogendiffusion into the particles also increases.

The net result of diffusion in and out of the particles during heattreatment determines the final hydrogen concentration. Thus, whenhydrogen is used as the motive gas, the particles produced by theprocess of this invention will still have some small amount of hydrogenremaining. All conditions being equal, this final hydrogen concentrationwill be greater than when some other motive gas not containing hydrogen,e.g. argon, is employed in the process.

As appreciated by a skilled practitioner, when a fluidized bed apparatusis used to conduct the process of this invention, a flow of gas impartsmotion to the particles being treated. When a moving bed is employed,particle motion is imparted by some other motive force, e.g. gravity.

To prepare polysilicon suitable for use in preparation of semiconductordevices, the process of this invention should be conducted underconditions which eliminate or substantially eliminate contamination ofthe polysilicon product. Thus, for example, the vessel in which thepolysilicon is heat treated should be one in which the particles areexposed only to high purity silicon and high purity fluids. Thus, it ispreferred that the parts of the vessel to which the particles areexposed, be fabricated from or coated with high purity silicon.

The polysilicon which is used as a starting material for the process ofthis invention, and which is made according to the process described inthe above-mentioned applications of Gautreaux and Allen typically has asurface dust content of about 0.1 weight percent or less. The process ofthis invention may reduce the surface dust content. In general, thereduction in surface dust is greater when higher operating temperaturesare used, and when the process of this invention is conducted whileusing a stream of motive gas to fluidize the polysilicon particles beingtreated.

With regard to the flow of motive gas, there is a threshold or minimumgas velocity required to keep the particle bed in a fluidized state.Operational velocities for this invention are generally somewhat abovethis minimum, U_(min). In many instances the operation velocity, U, is 1to 10 times U_(min) ; preferably U/U_(min) is 1.05 to 3.5.

EXAMPLE 1

Polysilicon of the type described in the above-cited applications of M.F. Gautreaux and Robert H. Allen was heated at a temperature of776.2.°±14.6° C. The initial hydrogen concentration in the polysiliconwas 619 ppma. The hydrogen concentration at various times is noted inthe following table.

                  TABLE II                                                        ______________________________________                                        Hours      H.sub.2 Content (ppma)                                             ______________________________________                                        1          579                                                                2          501                                                                3          458                                                                4          424                                                                5          403                                                                6          374                                                                ______________________________________                                    

The hydrogen content reducing process of this example can be conductedusing semiconductor grade, silane-derived, fluidized bed-producedpolysilicon composed of approximately spherical particles having a sizerange of 150/1500 microns, and a hydrogen content of 100/1000 ppma. Theprocess can be conducted for from 1 to about 6 hours using a temperaturewithin the range of about 1020° to about 1200° C., preferably at aprocess time of from about 2 to about 4 hours. The process is preferablyconducted while maintaining the particles in motion in a moving bedapparatus or a fluidized bed apparatus. When a fluidized bed method isused, the levitating gas can be selected from hydrogen, argon, neon,xenon, and similar non-contaminating inert gases. The process produces areduction in hydrogen content, for example below 30 ppma. Preferredpolysilicon produced by the process of this invention contains fromabout 5 to about 25 ppma hydrogen.

EXAMPLE 2

Twenty pounds of silane-derived, fluidized bed produced, semiconductorgrade polysilicon was charged to a fluidized bed reactor, and maintainedin a fluidized bed state using a levitation hydrogen flow of about 182.8SCFH. After the bed temperature reached 920° C., another eight pounds ofpolysilicon was charged.

The bed was maintained in fluidized motion at 910° C. for eight hours.Thereafter, a sample was taken. Analysis indicated that the surfacesilicon dust content was 0.006 weight percent. The surface dust value ofthe silicon charged to the reactor was 0.071 weight percent.

The hydrogen content of the polysilicon charged was reduced from 884ppma to 11 ppma.

As a further illustration of the process of this invention, a mass ofpolysilicon particles of 700 microns average diameter is fluidized withhydrogen using a hydrogen flow rate for fluidization of U/U_(min) =1.5.The bed temperature is kept at 1100° C. for an average particleresidence time of 12.67 hours.

The reactor surfaces exposed to the polysilicon being treated arecomposed of a non-contaminating substance such as high purity silicon.

The above process will reduce the hydrogen content of polysilicon from1000-1200 ppma to about 50 ppma. If the polysilicon charged has aninitial hydrogen concentration of about 600 ppma, the concentration willbe reduced to a value within the range of 20-25 ppma.

After reduction of the hydrogen content of the particles in the processdescribed above, the particles are cooled in a product cooler. Generallyspeaking, the particles are cooled to a temperature in the range of60°-65° C. prior to removal of the product to a product hopper.

As can be seen, the dehydrogenation process of this invention can be anoperation separate and distinct from the process in which thepolysilicon beads are formed. Furthermore, as can be seen by the abovedescription, dehydrogenation of polysilicon according to this inventioncan be conducted subsequent to the preparation of the polysilicon beads,which are to be dehydrogenated. A skilled practitioner having theabove-detailed description of the invention can make modifications andchanges of the embodiments described above, without departing from thescope or spirit of the invention as defined by the appended claims.

It will be readily apparent that this invention is susceptible toconsiderable variation in its practice within the spirit and scope ofthe appended claims, the forms hereinbefore, presented being merelyillustrative thereof.

We claim:
 1. A method for producing polysilicon particles having areduced hydrogen content, said process comprising:(a) formingpolysilicon particles which have a size within the size range of fromabout 150 to about 3000 microns, the formation being by a process whichcomprises thermally decomposing silane in a first fluidized bedcomprised of silicon particles whereby the silane metal produced fromthe thermal decomposition of the silane is deposited on the particles;and (b) subjecting the formed particles to heat in a second fluidizedbed in the presence of a non-containing inert gas for a time and at atemperature of from about 890° C. to about 1200° C., which temperatureexceeds the first fluidized bed temperature, whereby the combination ofthe time and the temperature in the second fluidized bed are sufficientto reduce the hydrogen content of the formed particles but insufficientto melt-consolidate the particles.
 2. The process of claim 1 wherein theformed particles have a size distribution of from about 400 to about1500 microns.
 3. The process of claim 1 wherein the temperature in thefirst fluidized bed is within the range of from about 590° C. to about650° C.
 4. The process of claim 1 wherein the temperature in the secondfluidized bed is within the range of from about 1020° C. to about 1200°C.
 5. The process of claim 1 wherein the temperature in the firstfluidized bed is within the range of from about 620° C. to 650° C. andwherein the temperature in the second fluidized bed is within the rangeof from about 1020° C. to about 1200° C.
 6. The process of claim 1wherein the time in the second fluidized bed is within the range of fromabout 1 to about 6 hours.
 7. The process of claim 1 wherein the formedparticles have an initial hydrogen content within the range of fromabout 100 to about 1000 ppma, the temperature within the first fluidizedbed is within the range of from about 500° C. to about 750° C., and thetemperature within the second fluidized bed is within the range of fromabout 1020° C. to about 1200° C.
 8. The process of claim 7 wherein thehydrogen content of at least one formed particle is reduced to a levelwhich is below 30 ppma hydrogen.
 9. A method for producing polysiliconparticles having a reduced silicon dust and hydrogen content, saidprocess comprising(a) forming polysilicon particles which have at leasta portion of their surface covered with fine silicon dust, the formationbeing by a process which comprises thermally decomposing silane in afirst fluidized bed comprised of silicon particles whereby the silanemetal produced from the thermal decomposition of the silane is depositedon the particles; and (b) subjected the formed particles to heat in asecond fluidized bed in the presence of a non-contaminating inert gasfor a time and at a temperature of from about 890° C. to about 1200° C.,which temperature exceeds the first fluidized bed temperature, wherebythe combination of the time and the temperature in the second fluidizedbed are sufficient to reduce the amount of fine silicon dust coveringthe particle surfaces and are sufficient to reduce the hydrogen contentof the formed particles but insufficient to melt-consolidate theseparticles.
 10. The process of claim 9 wherein the formed particles havea size distribution of from about 400 to about 1500 microns.
 11. Theprocess of claim 10 wherein the temperature in the first fluidized bedis within the range of from about 590° C. to about 650° C.
 12. Theprocess of claim 11 wherein the temperature in the second fluidized bedis within the range of from about 1020° C. to about 1200° C.
 13. Theprocess of claim 9 wherein the temperature in the first fluidized bed iswithin the range of from about 620° C. to 650° C. and wherein thetemperature in the second fluidized bed is within the range of fromabout 1020° C. to about 1200° C.
 14. The process of claim 9 wherein thetime in the second fluidized bed is within the range of from about 1 toabout 6 hours.
 15. The process of claim 10 wherein the formed particleshave an initial hydrogen content within the range of from about 100 toabout 1000 ppma, the temperature within the first fluidized bed iswithin the range of from about 500° C. to about 750° C., and thetemperature within the second fluidized bed is within the range of fromabout 1020° C. to about 1200° C.
 16. The process of claim 15 wherein thehydrogen content of at least one formed particle is reduced to a levelwhich is below 30 ppma hydrogen.
 17. In a process for preparingparticulate polysilicon, which process comprises: (a) a productivitymode in which silicon seed particles are contacted, in a fluidized bed,with a gas comprising from about 10 to about 100 mol % silane at atemperature which causes the thermal decomposition of the silane wherebysilicon metal resulting from such decomposition is deposited on the seedparticles to yield intermediate silicon particles and whereby silicondust is formed which deposits on the surfaces of such intermediatesilicon particles; and (b) a quality mode which comprises contacting, ina fluidized bed, the intermediate particle having surface dust with agas comprising from about 1 to about 5 mol % silane at a temperatureabove the thermal decomposition temperature of the silane whereby asilicon metal layer is deposited on the intermediate particles whichcements at least a portion of the silicon dust to the intermediateparticles, the improvement which comprises subjecting the intermediateparticles which have at least a portion of the silicon dust cementedthereto to heat in a fluidized bed in the presence of anon-contaminating inert gas for a time and at a temperature whichexceeds the fluidized bed temperatures used in forming such cementedintermediate particles so that the hydrogen content of such cementedintermediate particles is reduced, the temperature however beinginsufficient to melt-consolidate the cemented intermediate particles.18. The process of claim 17 wherein the cemented intermediate particleshave a size within the range of from about 150 to about 1500 microns.19. The process of claim 17 wherein the temperature used to reduce thehydrogen content of the cemented intermediate particles is within therange of from about 890° to about 1200° C.
 20. The process of claim 17wherein the temperatures in the fluidized beds used to form theintermediate particles and to cement the dust to the surfaces of theintermediate particles are within the range of from 500° to about 750°C.